1. Field of the Invention
The present invention relates to a method for cleaning a film formation apparatus.
2. Description of the Related Art
In recent years, researches on carbon materials such as a carbon nanotube, fullerene, graphene, and diamond which make full use of their features have been actively conducted all over the world. The carbon materials are industrially attractive in their features such as a small thermal expansion coefficient, high chemical resistance, and excellent electric conductivity and thermal conductivity.
Among these materials, attention has been particularly focused on graphene having a crystalline structure in which hexagonal skeletons of carbon are arranged in a plane form. Since graphene is one atomic plane extracted from a graphite crystal and has electrical, mechanical, or chemical characteristics which are outstanding, application of graphene has been expected in a variety of fields for, for example, a field-effect transistor with high mobility, a lithium ion secondary battery or a capacitor with high capacity and high output, a highly sensitive sensor, a highly-efficient solar cell, and a next-generation transparent conductive film, which use graphene. Although indium tin oxide (ITO) has been used for a transparent conductive film, there are growing concerns about depletion of indium, which is a constituent element, in the near future. For this reason, it is desirable to use carbon which is an abundant element on the earth as an alternative material.
Here, the graphene refers to one-atom-thick sheet of carbon molecules having π bonds or a stack in which a plurality of one-atom-thick sheets (the number of sheets is 2 to 100) of carbon molecules is stacked. In the case of the stack, an element other than carbon such as oxygen or hydrogen may be contained at a concentration less than or equal to 15 at. %.
As a method for industrially efficiently manufacturing such graphene, a thermal chemical vapor decomposition (CVD) method, in which graphene is formed over a metal catalyst, or the like has been attracting attention. The thermal CVD method is a kind of chemical vapor deposition (CVD) method, in which a thin film is formed using a decomposition product of a source gas caused by thermal energy, or chemical reaction. A metal catalyst is set in a reaction chamber heated to approximately 1000° C., and a hydrocarbon gas such as CH4 (methane) or C2H2 (acetylene) flows into the reaction chamber as a film formation gas, so that the hydrocarbon gas is thermally decomposed; thus, graphene can be formed over the metal catalyst. Since thermal decomposition is less likely to occur in a gas such as methane or acetylene, it is necessary to use a catalyst to promote decomposition of the film formation gas. As the metal catalyst, nickel (Ni), copper (Cu), or the like is used. With the use of the thermal CVD method, graphene can be uniformly formed over a substrate having a large area; therefore, the thermal CVD method is an industrially optimal method for manufacturing graphene.
However, the thermal decomposition reaction of the film formation gas not only takes place on a surface of an object (metal catalyst) but also proceeds to the entire region in the reaction chamber including an inner wall, a jig, and the like in the reaction chamber. As a result, a product of the film formation gas, specifically, carbon with an amorphous structure is deposited in a film form on the inner wall or the like of the reaction chamber. The deposited carbon film becomes a contamination source such as a particle or soot when the reaction chamber is used again. Moreover, the deposited carbon film falls on the object during film formation, in a flake form, which causes significant deterioration in quality of the film to be formed. Further, the carbon film deposited on the inner wall of the reaction chamber becomes a cause of disturbing the set environment inside the reaction chamber: for example, the temperature control for keeping the temperature inside the reaction chamber to be stable may be disturbed.
For these reasons, the product deposited on the inner wall of a reactor or the like needs to be removed by etching (hereinafter referred to as cleaning) periodically or every film formation. In a conventional cleaning method, in a small-sized film formation apparatus, a carbon film is manually removed after the inside of the reaction chamber is exposed to the air. However, as the size of the apparatus increases, it becomes difficult to operate manually; therefore, a method in which an oxygen gas is introduced into the reaction chamber, so that the carbon film is oxidized to be a COx gas and removed has been considered.
Patent Document 1 discloses a method of oxidizing and removing a substance attached to an inner wall or the like of a vacuum chamber after graphite nanofibers are formed by a thermal CVD method in a cold-wall type film formation apparatus which cools the wall of the vacuum chamber by a cooling medium provided in the periphery thereof. The attached substance is oxidized and removed by introducing 200-sccm air into the vacuum chamber and performing heat treatment at 600° C. for 2 minutes with an infrared lamp.